I think that the X6 3.5i intercooler would be the one to replace N54 intercooler with, or even the 740i intercooler. There is a thread on this forum with more details, there are not many modifications to be made to the X6 in order to fit.
Here is an idea: we should donate $10 per forum member to a trusted member who has fabrication skills and background in order to build a DIY kit to modify the X6 intercooler mountings/ducts to fit the 335i. The X6 intercooler costs about $200, I think that with $1000 cash someone could prototype such a kit.
Then we could buy an X6 intercooler and this ($100) modification kit and have a nice big OEM intercooler.